I just saw this story this morning: insurgency and thought of Michael Walzer's Just and Unjust Wars. While I don't have time to go into too much detail or properly cite, the issue at hand is legitimate authority. If legitimate authority is only ever taken to mean "the government," then revolutions against oppressive dictators are never just. Walzer nuances the idea of legitimate authority to include insurrection against an established government. He believes if the insurrection has enough popular support to be self-sustaining, it thus becomes a legitimate authority.
It doesn't appear that the Afghan insurgency has reached that level yet, or that it is anything other than a military organ (as opposed to having a political wing as well), but it is worth keeping an eye on.